A rapid and quantitative metod of imaging the three-dimensional in vivo distribution of radionuclides provides unique and valuable biochemical functional information. The proposed research is a collaborative effort among physicists, chemists, engineers, computer scientists and physicians to achieve a fundamental improvement in radionuclide imaging instrumentation. Focusing on radiolabelled monoclonal antibody imaging as a promising modality, the ultimate objective of the proposed research is to increase the sensitivity and quantitation accuracy of tumor imaging. Unlike conventional scintillation cameras which use mechanical collimation for imaging single photon emitting radionuclides and consequently suffer from an inherently low sensitivity since a large portion of the emitted radiation is absorbed by the lead septa, our proposed instrument provides more than an order of magnitude increase in sensitivity by using an electronic method of collimation. A sequential interaction of the emitted gamma rays with two position and energy sensitive detectors is used to localize activity upon conical surfaces within the body wherefrom the three-dimensional distribution can be reconstructed. The system design is based on an array of germanium detectors to serve as the first detector and a scintillation camera without its collimator as the second detector. Having successfully fabricated a unique 4x4 germanium detector for a prototype subsystem, we now propose to fabricate a 16x16 germanium detector and incorporate it into an instrument for preclinical evaluation studies of tumor imaging with radionuclides covering a wide range of energies. (Unlike mechanical collimation, electronic collimation is well suited to a wide energy range, from 100 keV to several MeV). Also, we propose to use modular scintillation cameras, instead of the conventional large-area scintillation camera, as the second detector. Modular cameras are essential to our design because the "singles" count rates are very high. Eventually, a clinical system comprising two orthogonal 32x32 germanium detectors and 16 scintillation camera modules will be implemented. A microcomputer based data acquisition circuit will be developed to record coincident counts between the germanium and the scintillation cameras. Computer simulation studies and experiments with various test objects and radionuclides will continue for developing appropriate algorithms to reconstruct three-dimensional emission images from the coincident counts.